Automatic grounding connection for electrical unit

ABSTRACT

The mounting screw for an electrical unit, such as a wall receptacle, is threaded through an electrically conductive tab and into the metal flush box so that as the screw is tightened, the tab is drawn toward the metal mounting strap or yoke to make an electrical conduction path between the screw and the mounting strap. In one embodiment, the screw is undercut so that when secured the tab is locked in place and cannot be unthreaded.

States Patent cHattie 1 1 Jan. 15, 19741 1 AUTOMATIC GROUNDINGCONNECTION FOR ELECTRICAL UNIT [76] Inventor: Earl E. Mcllattie, 2129Midlothian Rd., Roseville, Minn. 55113 [22] Filed: Aug. 18, 1971 [21]Applr No.: 172,728

[52] US. Cl 339/14 R [51] Int. Cl H01! 3/06 [58] Field of Search 339/14R, 133 R;

[56] References Cited UNITED STATES PATENTS 3,706,959 12/1972 Norden..339/14R 3,689,864 9/1972 3,663,919 5/1972 3,150,556 9/1960 Churchill85/32 3,432,793 3/1969 Muska et a1. 339/14 R 3,609,213 9/1971 Winter eta1. 3,543,634 12/1970 Allen 85/36 Primary Examiner-Marvin A. ChampionAssistant ExaminerRobert A. Hafer Att0rneyStryker and Jacobson [57]ABSTRACT The mounting screw for an electrical unit, such as a wallreceptacle, is threaded through an electrically conductive tab and intothe metal flush box so that as the screw is tightened, the tab is drawntoward the metal mounting strap or yoke to make an electrical conductionpath between the screw and the mounting strap. In one embodiment, thescrew is undercut so that when secured the tab is locked in place andcannot be unthreaded.

4 Claims, 5 Drawing Figures PATENTEUJAH 151914 v BLYSBLWQ INVENTOR EARLE. M 47775 AUTOMATIC GROUNDING CONNECTION FOR ELECTRICAL UNIT BACKGROUNDOF THE INVENTION 1. Field of the Invention This invention is directedtoward providing an automatic grounding path for a wall mountedelectrical unit such as a receptacle which is attached to a metal flushbox with suitable mounting screws.

2. Description of the Prior Art In U. S. Pat. No. 3,432,793, patentedMar. 11, 1969, titled Grounding Connection for Electrical Unit there isbriefly described the reason for having a ground connection. Asdescribed briefly therein, the combination of a three-wire cord, athree-prong plug and three socket receptacle protects the user of anelectrical apparatus, such as a household appliance, in the event one ofthe hot leads should short to the casing which is held by or in contactwith the user. Before the advent of the automatic grounding connectionwhich is described in the aforementioned patent, the usual way ofgrounding the third wire was to run a short piece of wire from thegrounding terminal on the electrical unit, such as a receptacle orswitch, to the metal flush box to which the unit was mounted. This wasnot only time consuming and costly, especially when one considers thelarge number of receptacles ordinarily found in residences and businessestablishments, but also it was sometimes overlooked by the tradesman inhis hurry to get his work done. By providing a connection of the naturedescribed in the Muska et al patent, supra, the grounding of the thirdwire is made automatically when the unit is mounted to the flush boxwith its mounting screws in the normal fashion. The present inventionserves the same function as that described in the aforementioned patent.

SUMMARY A small piece or tab of sheet metal contains a hole or openingthrough which it is threadably engaged with the mounting screw whichpasses through the metal mounting strap and is threaded into the metalflush box for mounting the electrical unit. The tab is arranged so thatwhen the unit is securely mounted, it makes electrical connectionbetween the screw and the mounting strap thereby insuring a goodelectrical connection between the ground wire of the unit, which isconnected through the receptacle to the mounting strap, and the flushbox. Preferably the tab is not attached to the mounting strap and islocated under the mounting strap so that when the screw is threaded intothe flush box, the tab is drawn into pressure contact with the mountingstrap. In that case, the screw is preferably undercut at its upper endto prevent the tab from unthreading if the screw is unthreaded from theflush box.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates in general the usualmanner of mounting a wall unit to its flush box to illustrate anembodiment of the present invention as used to complete the groundingcircuit between the unit and the flush box;

FIG. 2 is a view of a preferred embodiment of the automatic groundingmember;

FIG. 3 is a sectional view of the embodiment illustrated in FIG. 2;

FIG. 4 is another embodiment of the automatic grounding member; and

FIG. 5 is a view of the mounting screw used in the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now bedescribed as it is used in conjunction with an ordinary double,three-socket wall receptacle. It should be understood that the inventionis not so necessarily limited and can be used effectively with otherelectrical units such as wall switches and the like. An ordinary doublewall receptacle 10 contains three sockets (not shown) in each half formating with corresponding male prongs of an electrical plug which is atthe end of a three-wire cable which is normally attached to a piece ofelectrical equipment such as a household appliance. Two of the prongsand their mating sockets are used to provide the energizing current tothe appliance through the attached wires while the third prong providesa precautionary grounding path for the third wire. The receptacle hasthe usual rigid mounting strap or yoke 11 which is connectedelectrically to a grounding terminal 12 which is electrically connectedinternal of the receptacle, by means not shown, to the ground sockets ineach half of the receptacle. In the usual fashion, the strap 11 hasmounting ears 13 at each end which contain elongated slots 14 throughwhich mounting screws 15 are inserted to attach the receptacle 10 to ametal flush box 16 which is located in the wall. The flush box 16 isordinarily electrically grounded so that it provides a convenient placeto which to connect the grounding terminal 12. Various electrical codesand specifications require that resistance in the ground circuit path bevery low so that in the event a short should occur which would groundone of the power lines, a large current would be drawn almostinstantaneously to blow the protective fuse or circuit breaker or thelike. Furthermore, as a precaution against fire, it is generallyrequired that no sparking or arcing occur. It can be visualized thatbecause the metal strap 11 is made of relatively heavy metal and issolidly connected to the grounding terminal 12, it introduces negligibleresistance in the grounding path.

The present invention provides a member 17 to automatically complete alow resistance electrical circuit connection from the strap 111 to theflush box 16 through the mounting screw 15 when the receptacle ismounted or attached to the flush box but not making good electricalcontact with it. The tab member 17 is a thin piece of electricallyconductive material, preferably made from a sheet of metal, and, as willbe shown subsequently, more preferably a resilient metal such as springbronze. Except for certain requirements such as are set forth below, thesize and shape of the tab 17 is of no great consequence. The embodimentillustrated in FIG. 2 and 3 shows the tab 117 to be rectangular in planbut as viewed from an edge is bowed upward at two opposite ends. The tabcontains an opening 19 which is defined by four finger-like extensions20. Preferably the opening and the associated fingers are formed bypunching with a suitable die. When punched out of a piece of springmetal, the fingers 20 would appear in cross section substantially asillustrated in FIG. 3. The principal requirement as to the dimension ofopening 19 as defined by fingers 20 is that the opening be large enoughto accommodate the mounting screw but with fingers threadably engagedwith the screw threads. In order to ensure a good electrical contactbetween the fingers and the screw threads, it is also preferable thatthe dimensions be such that as the screw is threaded into the opening19, the fingers 20 are in pressure contact with the screw threads. Ineffect then, this means that it is preferred that the dimension of theopening between opposite fingers be less than the minor diameter of themounting screw.

In practice, the tab 17 is located so that the mounting screw 15 passesthrough the slot opening 14 in the mounting strap 11 and is threadedthrough opening 19 and into a suitably threaded hole in the flush box16. As the mounting screw advances until the unit or receptacle isapparently secured firmly to the flush box, the tab 17 is brought intopressure contact with strap 11 and through the fingers 20 makes a lowresistance electrical path between the strap 11 and the mounting screw15. Tab 17 may be located on the top side of the mounting strap 11 and,if so, the configuration of the fingers and the curvature of the tabwould of necessity be different from that illustrated in the drawing.However, it is preferred that the tab be located on the underside ofstrap 11 for the reason that it can then serve as a retainer for themounting screws. The former practice of packaging receptacles has beento have a cardboard washer under the strap and the screw insertedthrough the slot and through the washer which would then keep it inplace. This additional function would now be effectively served by tab17 being located under the mounting strap. The tab could be fixedlyattached to the top or bottom side of strap 11 in a manner to constitutea virtual duplicate of the invention in the aforementioned Muska et alpatent. However, using a tab of the fashion described hereinabove andattaching it to the mounting strap 11 has some drawbacks. First off, theslots 14 in the mounting strap 11 provide the tradesman flexibility intrying to align the mounting screws with the threaded openings in theflush box. A permanently fixed tab with a small opening 19 wouldsomewhat limit that flexibility. Secondly, if an attached tab were usedto hold the mounting screws in place before the unit is attached, asdescribed previously, the screws would have to be threaded substantiallyall the way in so they would not extend above the body of the receptacleso the receptacle would fit into its container. This, then, might alsointerfere with the freedom that the tradesman presently has in mountingthe unit and attaching it with the mounting screws to the flush box.

In the preferred embodiment where the tab is not attached to the strap,it is preferred that the shape and size of the tab be such that afterthe screw has been initially threaded into the opening 19, tab 17 needno longer be manually held in place to prevent it from rotating as thescrew is threaded into the flush box. This would be the case if the tabwere, for example, rectangular and dimensioned so that it would strikethe side of the receptacle body or the mounting strap when the screw isrotated and therefore be held in place and prevented from rotating sothat it will then be drawn down to the strap as the mounting screw isthreaded into the flush box. Also, in the preferred case where the tabis located on the underside of the mounting screw and is not attachedpermanently to the mounting strap, the mounting screw 15 shouldpreferably have a configuration as illustrated in FIG. 5. Just below thehead of the screw the shaft is undercut at 21. The undercut is to adegree that the remaining shaft has a diameter slightly less than theminor diameter of the screw. Constructed in this fashion, as the screwis threaded into the flush box and draws tab 17 until it makes pressurecontact with the underside of the mounting strap, when the fingers 20reach the undercut area 21, they are then located so that the tab 17cannot be unthreaded from the screw. Of course, in that situation thefingers should still be in pressure contact with the upper part of thetopmost thread to make a good electrical connection to the screw. Thiscan be achieved by the proper curvature or bow of the tab combined withthe degree to which the fingers extend downward from the underside ofthe tab. The reason for this is that if the receptacle is once mountedin place and then must be removed for some reason, it is preferred thatthe tab 17 not be unthreaded as the mounting screw is unthreaded fromthe flush box as would likely be the case if the undercut area in thescrew were not provided. Naturally, the length of the undercut area mustbe such that, as stated before, there is still pressure contact betweenthe fingers 20 and the screw thread.

An illustration of a possible variation in construction of the tab 17 isshown in FIG. 4. A relatively flat piece of metal or tab has upwardextending legs 22 pierced or otherwise formed out of the tab body.Again, screw 15 would be threadably engaged in an opening 20 so that asthe screw is threaded into the flush box, the tab is drawn toward themounting strap until the legs 22 make pressure contact therewith. Inthis embodiment, it is again preferred that the opening be defined byfingers in the same manner as described hereinbefore.

In a typical case, a satisfactory grounding connection was made mountinga double wall socket to the flush box using a rectangular tab made outof spring bronze of approximately 0.015 thickness with the opening 19stamped out of the tab material so that the fingers define an openinghaving a maximum dimension between opposite fingers in the order of0.092 0.093 inch. The mounting screw was a six thirty-second which has aminor diameter of 0.0974 and the length of the undercut extended about0.030 inch from below the screw head and the diameter of the screw shaftin the undercut area was in the order of 0.091 0.092 inch. The radius ofcurvature of the tab was not determined but was sufficient so that asthe tab was drawn down into pressure contact with the underside of themounting strap and the fingers of the tab slipped into the undercut areaof the mounting screw the fingers maintained good pressure contact withthe screw body.

I claim:

1. For an electrical unit such as a wall receptacle which has a metalmounting strap for attaching the unit to a metal flush box, apparatusfor making electrical connection between the mounting screw and thestrap, comprising:

a resilient tab of electrically conductive material separate from themetal strap and located under the metal strap having an opening definedby inwardly extending resilient fingers for receiving a threaded member;said tab being bent so at least one edge curves upward toward the strap,the mounting screw passing through the strap and threadably engagingsaid resilient fingers and biasing said fingers in the direction ofadvance of the screw so that as the screw is threaded into the box thetab is drawn upward to make pressing low resistance electrical contactwith the strap, the shank of said mounting screw containing acircumferential unthreaded relief section just below the screw headsomewhat deeper than to the minor diameter of the threaded sectionwhereby when the screw is threaded through the opening in the tab beyondthe end of the threads, the fingers come to rest in the relief sectionwhile still making electrical contact with the screw.

2. The invention as set forth in claim 1 wherein the width of said tabis such that as the screw is threaded through the opening in the tab, anedge of the tab strikes the mounting strap or the receptacle body toprevent the tab from rotating.

3. In combination:

an electrical unit such as a wall receptacle having a metal mountingstrap;

a mounting screw passing through an opening in said strap;

a resilient electrically conductive tab separate from said strap andlocated under the strap having an opening defined by inwardly extendingfingers threadably engaging the screw threads, said fingers being biaseddownward by said screw threads;

said tab being bent so at least one edge curves upward toward the strapwhereby when the unit is mounted with said mounting screw the tab makespressure contact with the strap to establish low electrical resistanceconnection between the strap and the screw;

the shank of said mounting screw containing a cir- 4. The invention asset forth in claim 3 wherein the width of the tab is such that as thescrew is threaded through the opening in the tab an edge of the tabstrikes the mounting strap or the receptacle body to prevent the tabfrom rotating.

1. For an electrical unit such as a wall receptacle which has a metalmounting strap for attaching the unit to a metal flush box, apparatusfor making electrical connection between the mounting screw and thestrap, comprising: a resilient tab of electrically conductive materialseparate from the metal strap and located under the metal strap havingan opening defined by inwardly extending resilient fingers for receivinga threaded member; said tab being bent so at least one edge curvesupward toward the strap, the mounting screw passing through the strapand threadably engaging said resilient fingers and biasing said fingersin the direction of advance of the screw so that as the screw isthreaded into the box the tab is drawn upward to make pressing lowresistance electrical contact with the strap, the shank of said mountingscrew containing a circumferential unthreaded relief section just belowthe screw head somewhat deeper than to the minor diameter of thethreaded section whereby when the screw is threaded through the openingin the tab beyond the end of the threads, the fingers come to rest inthe relief section while still making electrical contact with the screw.2. The invention as set forth in claim 1 wherein the width of said tabis such that as the screw is threaded through the opening in the tab, anedge of the tab strikes the mounting strap or the receptacle body toprevent the tab from rotating.
 3. In combination: an electrical unitsuch as a wall receptacle having a metal mounting strap; a mountingscrew passing through an opening in said strap; a resilient electricallyconductive tab separate from said strap and located under the straphaving an opening defined by inwardly extending fingers threadablyengaging the screw threads, said fingers being biased downward by saidscrew threads; said tab being bent so at least one edge curves upwardtoward the strap whereby when the unit is mounted with said mountingscrew the tab makes pressure contact with the strap to establish lowelectrical resistance connection betweeN the strap and the screw; theshank of said mounting screw containing a circumferential unthreadedrelief section just below the screw head somewhat deeper than to theminor diameter of the threaded section whereby when the screw isthreaded through the opening in the tab beyond the end of the threadsthe fingers come to rest in the relief section while still makingelectrical contact with the screw.
 4. The invention as set forth inclaim 3 wherein the width of the tab is such that as the screw isthreaded through the opening in the tab an edge of the tab strikes themounting strap or the receptacle body to prevent the tab from rotating.